Alveolar bone

Three distinct areas can be seen:

1. Alveolar bone proper (bone socket or crypt)
Spongy bone: that fills the space between alveolar bone proper and compact
bone. ( filled with spaces that were occupied by bone marrow cells , spongy
bone makes the alveolar bone lighter and provides blood supply )

2. Compact bone or cortical plates: which form the outer and inner plates of the
alveolar process
NOTE : in the maxilla cortical bone is thin bucally , in the mandible cortical bone
is thin lingually
3. Mandibular bone / maxillary bone

spongy +compact bone = supporting alveolar bone.

No teeth à alveolar bone resorption ( you wont be able to put a denture or prosthesis or place an
implant )

Alveolar bone proper : It has many names such as:

• Bone lining socket: lines the tooth sockets of the alveolus

• Cribriform Plate: contains numerous holes representing the
Volkmann's canals of the bone.
• Bundle bone: provides attachment site for the sharpey’s fibers.

bone lining socket = cribriform plate = bundle bone = lamina dura

why is it called bundle bone ? because of sharpey’s fibers

• The bundle bone appears radiographically more dense than the

adjacent supportive bone and is called the lamina dura.

Q:Why does it appear more radiopaque ?

A: The higher density of the lamina dura is due to the presence of thick bone or
due to the orientation of the bone crystals around the bundles of the fiber the
radio opacity is not because of increased mineral content.

Alveolar crest: the most cervical edge or rim of the alveolar bone proper

when we don’t see lamina dura à bone resorption ( ex: periodontitis ).

Recall : Dentin has 70 % inorganic and 30% organic /Cementum had 55% inorganic and 45% organic

Done By : Sima Habrawi 1

 Composition of bone
Organic ( 33 % ) Inorganic ( 67%)
Collagen – type 1 ( 25 % ) Hydroxyapatite
Non collagenous protein ( important in
mineralization) ( 8%)
Ex:
1) Osteocalcin
2) Osteopontin
3) Osteonectin
4) Sialoprotein
5) Phosphoprotein
6) BMP

Bone functions:

1) Bone supports and protects vital organs. Alveolar bone

supports and protects teeth.
2) Bone contains bone marrow, which produce blood cells.
3) It also serves as a reservoir of calcium, phosphate, and
other ions.
4) Bone and skeletal muscles are important for body
movements.
5) Bone as other type of connective tissue is consisting of
cells, fibers and ground substance.

Bone cells: Other cells :

1) Osteocytes 1. Undifferentiated cells
2) Osteocytes 2. Fibroblasts
3) Osteoclast 3. Macrophages
4. Blood cells

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BONE CELLS
TYPE Osteoblast
ORIGIN Undifferentiated mesenchymal cells ( progenitor osteoblasts)
SHAPE are flattened or cuboidal active cells line the surfaces of a developing bone as a
continuous layer surrounding the bone marrow.

LOCATION line the surfaces of a developing bone as a continuous layer surrounding the bone marrow.

FUNCTION
Osteoblasts have the ability to form the bone matrix by synthesizing and secreting collagen
type I protein, protoglycans and glycoproteins The calcium phosphate is then added to
calcify the matrix.

secretes a variety of cytokines that regulate bone remodeling (Osteoprotegerin (OPG) &
receptor-activated Nuclear Factor –kappa B ligand (RANKL)

Recall : cytokines are chemicals that affect nearby cells while hormones are chemicals that
affect far away cells .

NOTES When osteoblasts are active they will have very extensive rER , Golgi Apparatus ,When it is
non active it becomes very thin and it will be called bone lining cell.
¾ Both pre-osteoblasts and osteoblasts exhibit high levels of alkaline phosphatase (
enzyme that is responsible for mineralization )
¾ Adjacent cells are connected by gap junctions.
¾ Two transcription factors are essential for osteoblast differentiation from stem
cells and for their function;
Runx2 and Osterix.
¾ Mice that do not express Runx2 or Osterix show complete absence of bone
formation.

• Osteoprotegrin works to control osteoclasts and prevent excessive resorption. OPG works
against RANKL.
• RANKL & OPG are produced by osteoblasts + lymphocytes + PDL fibroblasts.
• RANKL activates osteoclasts by binding to surface receptros (RANK).
• OPG competitively binds to RANKL which inhibits RANKL activation of osteoclast formation.

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Q: what are the things needed for mineralization of bone?

1. Blood vessels ( to carry calcium and phosphate ions )

2. Non collagenous proteins
3. Alkaline phosphatase
¾ Resorption > formation à thin bone ( osteoporosis )
¾ Formation > resorption à normal in growing but if it is excess you’ll have dense bone
Osteoporosis affects all people after 50 but mostly affects postmenopausal women (why?)
Post menopause the level of estrogen reduces

Normally : estrogen increases the formation of OPG ( which prevents RANKL from binding to RANK and

BONE CELLS
TYPE Osteocyte
ORIGIN Osteoblast trapped in their own matrix
SHAPE occupying spaces called lacunae + The cytoplasmic processes
pass through fine canals called canaliculi
LOCATION Inside bone matrix ( osteoid )
FUNCTION maintaining bone and providing it with nutrient + proposed to
participate in local degradation of bone.

BONE CELLS
TYPE Osteoclast
ORIGIN Haematopoietic precursors of monocyte/macrophage by
macrophage colony-stimulating factor (M-CSF).

SHAPE multinucleated giant cell (50-100 µm)

FUNCTION Bone resorption
NOTES Motile cell due to microtubules
When the cell is moving it is not polarized when it is attached
to bone it is polarized
Osteoclast is characterized cytochemically by TRAP (Tartrate-
resistant acid phosphatase) staining.

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Q:How can you differentiate between macrophage and osteoclasts?

A: (TRAP) will color osteoclast red.

NOTE : osteoblasts can be detected by alkaline phosphatase

In order for monocytes to become pre-osteoclasts they need Macrophage colony stimulating factor
MCSF, they will later be activated by the attachment of RANKL , ( if RANKL is not attached to the
osteoclast it remains inactive )

An active osteoclast shows three distinct

area:

1- the ruffled border

2- the basolateral membrane

3- the sealing zone

Q:What controls what the undifferentiated cells become ? The culture media ( growth factors)

Bone resorption requires :

• Osteoclasts attach to the bone by the integrin called ( vetronectin )

• Polarization of osteoclasts ( attachment to bone )
• The sealing zone ( clear zone ) is called this way because it only contains mitochondria - Sealing
zone mediates the tight attachment of osteoclast to bone in order to maintain the acidic pH of
resorption lacunae.
• Carbonic anhydrase II + H+ (proton) pump on the ruffled border acidifies resorption lacuna and
dissolves mineral (bone crystals)
• The acidic medium it important to activate the lysosomal enzymes and dissolve the crystals of
bone à And expose the matrix

NOTE : the bone dissolves first before the lysosomal enzymes are activated.

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Molecular Mechanism of Bone Resorption :
• Osteoclast secretes lysosomal enzymes such as acid phosphatase, collagenase and
cathepsins extracellularly into resorption lacunae.
• These secreted enzymes are targeted to the ruffled-border by mannose-6-phosphate
receptors (MPR).
• The resorption lacunae is acidified by proton pump from the ruffled border and sealed off by
the attachment of osteoclast to bone by sealing zone.
• The mineral phase dissolves in the low pH, exposing the organic matrix to lysosomal
enzymes that are activated by the acidic milieu which results in digestion of collagenous and
non-collagenous proteins.

NOTE : males do have estrogen in enough levels to form OPG

Para Thyroid Hormone and vitamin D

Q:PTH and vitamin D work opposite to each other in cases and work together in other cases . Explain ?

A: High dose of vitamin D will increase bone resorption ( acts the same way as PTH ) , but in normal
cases will will act against PTH ( will aid in bone formation ).

PTH and vitamin D ensure calcium

homeostasis , because excess CA will
result in tetanus

Increased PTH and Vitamin D level

leads to increase serum calcium levels

1,25-dihydroxyvitamin D3 (Vitamin D, calcitriol) is the active form that controls calcium homeostasis
in body by targeting intestines and bones

1. ↑ serum Ca2+ and phosphate via

2. ↑ absorption of calcium and phosphate from the intestine
3. ↑ bone resorption of Ca2+ and phosphate

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 PTH functions to ↑ serum Ca2+ but ↓ serum phosphate:

- PTH stimulates 1,25-(OH)2 vitamin D production

- hypocalcemia/hypophoshatemia stimulates 1,25-(OH)2 vitamin D production
- 1,25-(OH)2 vitamin D feedback negatively on itself

Regulation of osteoclasts :

Q:Why are c- cells called this way ? where are they found ? what is their function ?

A:Because they are called clear cells since they don’t take the stain in histological sections , they are
found in the thyroid gland , function : produce calcitonin hormone .

Postmenopausal women don’t take estrogen they take calcitonin because estrogen can cause cancer

PTH Parathyroid hormone activates and increases the number of osteoclasts. It

increases secretion of RANKL and suppress OPG production.

Estrogen increases secretion of OPG and suppress RANKL production which inhibits
matrix resorption.
Calcitonin inhibits matrix resorption. The hormone calcitonin is secreted by C-cells of
thyroid gland.

Cytokines Immune cell products such as interleukin-1 and the tumor necrosis factors
(TNF) are stimulators of bone resorption in vitro.
1,25,dihydroxyvitamin the active form of vitamin D- increases the absorption of Ca and P and inhibits
D3 release of PTH.

Calciotropic factors à increase bone

resorption ( they increase the
production of RANKL from osteoblasts ,
lymphocytes and fibroblasts ) à RANKL
will bind to OPG and the excess will
bind to osteoclasts to activate it and
start bone resorption.

PGE2 : prostaglandin 2 à activates Pain

TNF : tumor necrotic factor
Prolactin : lactating hormone

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 Anabolic factors à increase the
formation of OPG by osteoblasts ,
fibroblasts , and lymphocytes à OPG
will bind to RANKL inhibiting it’s
activity and preventing it from binding
to osteoclasts and prevent it from
resorbing bone.

BMP : bone morphogenetic protein (

also found in dentin )

Clinical considerations :

• When teeth are lost the alveolar process undergoes gradual atrophy. 9 due to old age ,
trauma or perio diseases )
• In some cases sever resorption occurs so it is difficult to make prosthesis to replace the lost
teeth.
• The rate of bone remodeling is very fast in young children and decreases with age.
• Bone remodeling is related to several factors: hormones; growth factors and stress caused
by tooth movements.
• Bone resorbed on the side of pressure and new bone formed at the side of tension that
enables the orthodontic movement of teeth.
• At the pressure side there are increase in the level of cyclic adenosine monophosphate
(cAMP) in cells, which increases the osteoclasts activities
• Bone resorption is associated with periodontal diseases due to the presence of bacteria and
the presence of a peptide called Osteoclast Activating Factor (OAF), which is now called IL-
1, in the lymphocytes near the periodontal pocket.
• Alveolar bone loss is difficult to control or repair so this represent a great challenge in
periodontology.
• If there are excessive forces à first the cementum will resist then it will undergo resorption

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